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Haloalkanes and Haloarenes: one-page cheat sheet

NEET Chemistry · Unit 16 · Organic Chemistry

ScoreChemby CSJ Sir
Weightage: 2–3 questions · 8–12 marks

1 · Classification & C–X bond

s p^{3}

C–Xalkyl, allylic (next to C=C), benzylic (next to ring)

s p^{2}

C–Xvinylic and aryl halides

Bond	Length (pm)	Enthalpy (kJ)
C–F	139	452
C–Cl	178	351
C–Br	193	293
C–I	214	234

DipoleCH

_{3}

>

_{3}

>

_{3}

>

_{3}

b.p.RI

>

RBr

>

RCl

>

RF; branching lowers b.p.; para isomer: highest m.p.

2 · Preparation

SOCl

_{2}

best lab route; SO

_{2}

and HCl escape

PCl

_{5}

/ PBr

_{3}

give POCl

_{3}

/ H

_{3}

_{3}

as by-products

LucasHCl / ZnCl

_{2}

3^{\circ}

fastest

FinkelsteinRCl or RBr + NaI (dry acetone)

\to

SwartsRBr + AgF (or Hg

_{2}

_{2}

)

\to

SandmeyerArN

_{2}^{+}

+ Cu

_{2}

_{2}

\to

ArX

ArIArN

_{2}^{+}

+ KI, no copper needed

Balz–SchiemannArN

_{2}^{+}

_{4}^{-}

(

Δ

)

\to

ArF

3 · S $_{N}$ 1 vs S $_{N}$ 2

	S $_{N}$ 2	S $_{N}$ 1
Rate	$k [RX] [Nu]$	$k [RX]$
Steps	one, concerted	two, via cation
Substrate	$CH_{3} > 1^{\circ} > 2^{\circ} > 3^{\circ}$	$3^{\circ} >$ benzyl, allyl $> 2^{\circ}$
Product	inversion	racemisation
Solvent	polar aprotic	polar protic
Nucleophile	strong	weak is enough

Leaving groupI

^{-}

>

^{-}

>

^{-}

≫

^{-}

(both)

4 · Stereochemistry

Chiralfour different groups; mirror image not superimposable

Enantiomersequal and opposite rotation; same m.p. and b.p.

Racemic1:1 mixture, net rotation zero

Mesochiral centres plus a mirror plane; inactive

Diastereomersnot mirror images; different properties

5 · Elimination & metals

alc. KOH

β

-elimination: Saytzeff (more substituted alkene)

aq. KOHsubstitution gives the alcohol

GrignardRX + Mg (dry ether)

\to

RMgX; water gives RH

Wurtz–FittigArX + RX + 2Na

\to

ArR

AmbidentKCN gives RCN; AgCN gives RNC (isocyanide); AgNO

_{2}

gives RNO

_{2}

6 · Haloarenes

Why unreactiveresonance (C–Cl 169 pm),

s p^{2}

carbon, unstable phenyl cation,

π

repulsion

Dow processPhCl + aq. NaOH, 623 K, 300 atm

Substrate	NaOH temp.
4-nitrochlorobenzene	443 K
2,4-dinitrochlorobenzene	368 K
2,4,6-trinitrochlorobenzene	warm water

EASweakly deactivating (

- I

), ortho/para-directing (

+ M

)

7 · Polyhalogens

CHCl

_{3}

air and light give phosgene COCl

_{2}

; store in full, dark bottles with 1% ethanol

CHI

_{3}

yellow antiseptic

CCl

_{4}

fire extinguisher; phosgene with hot steam

Freon-12CCl

_{2}

_{2}

(Swarts); Cl radicals deplete ozone

DDTchlorobenzene + chloral; persistent, biomagnified

8 · Reagents on RX

Reagent	Product
aq. KOH	ROH
alc. KOH	alkene
KCN / AgCN	RCN / RNC
KNO $_{2}$ / AgNO $_{2}$	R–ONO / R–NO $_{2}$
NH $_{3}$	RNH $_{2}$
Na, dry ether	R–R
Mg, dry ether	RMgX

Seen in NEET 2024

Fastest S $_{N}$ 1: bromobenzene, 1-bromo-1-phenylethane, (bromomethyl)cyclohexane, bromocyclohexane.

1-Bromo-1-phenylethane: it gives a resonance-stabilised benzylic cation.

Seen in NEET 2025, 2021

1-Iodobutane is faster in S $_{N}$ 2 than 1-chlorobutane: I $^{-}$ is the better leaving group.

Bond enthalpy: CH $_{3}$ F $>$ CH $_{3}$ Cl $>$ CH $_{3}$ Br $>$ CH $_{3}$ I.

Seen in NEET 2020, 2022, 2023

Aryl halides resist S $_{N}$ : resonance, $s p^{2}$ carbon and phenyl cation instability, all together.

Enantiomers are non-superimposable mirror images.

CH $_{3}$ CH=CHCH(Cl)CH $_{2}$ CH $_{3}$ is an allylic halide.

Seen in NEET 2026

Propan-1-ol + PCl $_{5}$ gives 1-chloropropane, POCl $_{3}$ and HCl.

C $_{2}$ H $_{5}$ Cl + AgCN gives ethyl isocyanide, C $_{2}$ H $_{5}$ NC.

Six traps to remember

SOCl $_{2}$ is the best route from alcohols: both by-products are gases.
S $_{N}$ 2 inverts the configuration; S $_{N}$ 1 racemises it.
Alc. KOH eliminates; aq. KOH substitutes.
AgCN gives the isocyanide, KCN gives the cyanide.
Haloarenes resist nucleophiles; ortho/para nitro groups activate them.
Chloroform is stored in dark, full bottles to avoid phosgene.

Haloalkanes and Haloarenes: free one-page cheat sheet